Edmontosaurus was one of the largest hadrosaurids, measuring up to 13 meters (43 ft) long and weighing around 4.0 metric tons (4.4 short tons). It is known from several well-preserved specimens that include not only bones, but in some cases extensive skin impressions and possible gut contents. It is classified as a hadrosaurine hadrosaurid (those hadrosaurids which lacked a hollow crest), and was closely related to Anatotitan, if not a synonym.

Edmontosaurus has a lengthy and complicated taxonomic history dating to the late 19th century. Various species classified with genera such as Claosaurus, Thespesius, Trachodon, and the well-known but now defunct genus Anatosaurus are now regarded as belonging to Edmontosaurus. The first fossils named Edmontosaurus were discovered in southern Alberta, Canada, in what used to be called the lower Edmonton Formation. The type species, E. regalis, was named by Lawrence Lambe in 1917, although several other species that are now classified in Edmontosaurus were named earlier. The best known of these is E. annectens, originally named by Othniel Charles Marsh in 1892 as Claosaurus annectens and known for many years as Anatosaurus annectens. A third smaller species, E. saskatchewanensis, is also known. The name Edmontosaurus means "Edmonton lizard"; the genus was named after the Edmonton Formation, now known as the Horseshoe Canyon Formation.

Edmontosaurus was widely distributed across western North America. The distribution of Edmontosaurus fossils suggests that it preferred coasts and coastal plains. It was an herbivore that could move on both two legs and four. Because it is known from several bone beds, Edmontosaurus is thought to have lived in groups, and may have been migratory as well. The wealth of fossils has allowed researchers to study its paleobiology in detail, including its brain, how it may have fed, and its injuries and pathologies, such as evidence for a tyrannosaur attack on one edmontosaur specimen.

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Edmontosaurus has been described in detail from several specimens.[2][3][4][5] Like other hadrosaurids, it was a bulky animal with a long, laterally flattened tail and a head with an expanded, duck-like beak. The skull had no bony crest, unlike many other hadrosaurids, but bore a comb-like crest of skin and scales. The fore legs were not as heavily built as the hind legs, but were long enough to be used in standing or movement. Edmontosaurus was among the largest hadrosaurids: depending on the species, a fully grown adult could have been 9 metres (30 ft) long, and some of the larger specimens reached the range of 12 metres (39 ft)[6] to 13 metres (43 ft) long.[7] Its weight was on the order of 4.0 metric tons (4.4 short tons).[8] Traditionally, E. regalis has been regarded as the largest species, though this was challenged by the hypothesis that the larger hadrosaurid Anatotitan copei is a synonym of Edmontosaurus annectens, as put forward by Jack Horner and colleagues in 2004,[8] and supported in studies by Campione and Evens in 2009 and 2011.[9] The type specimen of E. regalis, NMC 2288, is estimated as 9 to 12 metres (30 to 39 ft) long.[10] E. annectens is often seen as smaller. Two well-known mounted skeletons, USNM 2414 and YPM 2182, measure 8.00 metres (26.25 ft) long and 8.92 metres (29.3 ft) long, respectively.[10][11] However, these are probably subadult individuals,[9] and there is at least one report of a much larger potential E. annectens specimen, almost 12 metres (39 ft) long.

The skull of a fully grown Edmontosaurus could be over a metre long. One skull of E. annectens (formerly Anatotitan) measures 3.87 feet (1.18 m) long.[13] The skull was roughly triangular in profile,[2] with no bony cranial crest.[14] Viewed from above, the front and rear of the skull were expanded, with the broad front forming a duck-bill or spoon-bill shape. The beak was toothless, and both the upper and lower beaks were extended by keratinous material.[8] Substantial remains of the keratinous upper beak are known from the "mummy" kept at the Senckenberg Museum.[6] In this specimen, the preserved nonbony part of the beak extended for at least 8 centimetres (3.1 in) beyond the bone, projecting down vertically.[15] The nasal openings of Edmontosaurus were elongate and housed in deep depressions surrounded by distinct bony rims above, behind, and below.[16] In at least one case (the Senckenberg specimen), rarely preserved sclerotic rings were preserved in the eye sockets.[17] Another rarely seen bone, the stapes (the reptilian ear bone), has also been seen in a specimen of Edmontosaurus.[8]

Teeth were present only in the maxillae (upper cheeks) and dentaries (main bone of the lower jaw). The teeth were continually replaced, taking about half a year to form.[18] They were composed of six types of tissues, rivaling the complexity of mammal teeth.[19] They grew in columns, with an observed maximum of six in each, and the number of columns varied based on the animal's size.[5] Known column counts for the two species are: 51 to 53 columns per maxilla and 48 to 49 per dentary (teeth of the upper jaw being slightly narrower than those in the lower jaw) for E. regalis; and 52 columns per maxilla and 44 per dentary for E. annectens (an E. saskatchewanensis specimen).

The number of vertebrae differs between specimens. E. regalis had thirteen neck vertebrae, eighteen back vertebrae, nine hip vertebrae, and an unknown number of tail vertebrae.[14] A specimen once identified as belonging to Anatosaurus edmontoni (now considered to be the same as E. regalis) is reported as having an additional back vertebra and 85 tail vertebrae, with an undisclosed amount of restoration.[14] Other hadrosaurids are only reported as having 50 to 70 tail vertebrae,[8] so this appears to have been an overestimate. The anterior back was curved toward the ground, with the neck flexed upward and the rest of the back and tail held horizontally.[8] Most of the back and tail were lined by ossified tendons arranged in a latticework along the neural spines of the vertebrae. This condition has been described as making the back and at least part of the tail "ramrod" straight.[20][21] The ossified tendons are interpreted as having strengthened the vertebral column against gravitational stress, incurred through being a large animal with a horizontal vertebral column otherwise supported mostly by the hind legs and hips.

The shoulder blades were long flat blade-like bones, held roughly parallel to the vertebral column. The hips were composed of three elements each: an elongate ilium above the articulation with the leg, an ischium below and behind with a long thin rod, and a pubis in front that flared into a plate-like structure. The structure of the hip hindered the animal from standing with its back erect, because in such a position the thigh bone would have pushed against the joint of the ilium and pubis, instead of pushing only against the solid ilium. The nine fused hip vertebrae provided support for the hip.[5]

The fore legs were shorter and less heavily built than the hind legs. The upper arm had a large deltopectoral crest for muscle attachment, while the ulna and radius were slim. The upper arm and forearm were similar in length. The wrist was simple, with only two small bones. Each hand had four fingers, with no thumb (first finger). The index second, third, and fourth fingers were approximately the same length and were united in life within a fleshy covering. Although the second and third finger had hoof-like unguals, these bones were also within the skin and not apparent from the outside. The little finger diverged from the other three and was much shorter. The thigh bone was robust and straight, with a prominent flange about halfway down the posterior side.[5] This ridge was for the attachment of powerful muscles attached to the hips and tail that pulled the thighs (and thus the hind legs) backward and helped maintain the use of the tail as a balancing organ.[22] Each foot had three toes, with no big toe or little toe. The toes had hoof-like tips.

Multiple specimens of Edmontosaurus annectens have been found with preserved skin impressions. Several have been well-publicized, such as the "Trachodon mummy" of the early 20th century,[23][24] and the specimen nicknamed "Dakota",[25][26][27] the latter apparently including remnant organic compounds from the skin.[27] Because of these finds, the scalation of Edmontosaurus annectens is known for most areas of the body. Skin impressions are less well known for E. regalis, but some well-preserved examples have been studied, including one which preserves a soft tissue crest or wattle on the head. It is unknown whether such a crest was present on E. annectens, and whether it was an indicator of sexual dimorphism.[1]

AMNH 5060, the "Trachodon mummy" (so-called because it appears to be a fossil of a natural mummy), is a well-known specimen now classified as E. annectens. It was found to have skin impressions over the snout, much of the neck and torso, and parts of the arms and legs.[15] The tail and part of the legs eroded before collection, so these areas are unknown for the specimen.[24] Additionally, some areas with skin impressions, such as sections associated with the neck ridge (see below) and hands, were accidentally removed during preparation of the specimen.[23] The specimen is thought to have desiccated in a dry stream bed,[24] probably on or near a point bar. The circumstances of the location and preservation of the body suggest that the animal died during a prolonged drought, perhaps from starvation.[28][29] The desiccated carcass was eventually buried in a sudden flood, surrounded by sediment that had enough fine particles to make a cast of the epidermal structures.

The epidermis was thin, and the scalation composed of small nonoverlapping scales,[23] as seen in the Gila monster.[15] Two general types of scales were present over most of the body: small pointed or convex tubercles, 1 to 3 millimetres (0.039 to 0.118 in) in diameter with no definite arrangement (ground tubercles); and larger, flat polygonal tubercles (pavement tubercles) typically less than 5 millimetres (0.20 in) in diameter, but up to 10 millimetres (0.39 in) over the forearm. The pavement tubercles were grouped into clusters separated by ground tubercles, with transitional scales between the two types. Over most of the body, the pavement tubercles were arranged in circular or oval clusters, while near the shoulder on the upper arm, they formed strips roughly parallel to each other and the shoulder blade. Generally, clusters were larger on the upper surfaces of the body and smaller on the underside. Clusters up to 50 centimetres (20 in) in length were present above the hips.

Impressions from the head came from the large opening for the nostrils. Instead of tubercle impressions, there were impressions of folded soft tissue, with a deeper area at the anterior end of the opening that may have been the approximate location of the nostril itself.[15] The neck and back had a soft ridge or frill running down the midline, with a row of oval tubercle clusters arranged above the spines of the vertebrae. The total height of the ridge on AMNH 5060 is not known, nor the disposition of its upper border, as the upper extremity was prepared away. The ridge was at least 8 centimetres (3.1 in) tall, and was folded and creased to permit movement. Osborn proposed that it was tall enough for another row of clusters.[24]

The forearms had the largest tubercles, arranged in single large clusters that covered the leading surfaces. The hands were covered in small pavement tubercles in a soft-tissue structure than enclosed the three central fingers; not even the tips were exposed. Osborn interpreted this as a paddle for swimming.[24] Robert T. Bakker later reinterpreted it as a soft-tissue pad for walking, analogous to that of a camel.[30] Like the forearm, the shin had large tubercles. The scalation of the rest of the leg is not presently known, although impressions on a specimen of the hadrosaurid Lambeosaurus suggest that the thighs were under the skin of the body, like modern birds.[15]

The tail of AMNH 5060 was not present, but other specimens have filled in some details for that area. Skin impressions from a partial tail belonging to Edmontosaurus annectens, recovered from the Hell Creek Formation of Montana, show a segmented ridge above the vertebrae. The ridge was about 8.0 centimetres (3.1 in) tall, with the segments being about 5.0 centimetres (2.0 in) long and 4.5 centimetres (1.8 in) high, spaced 1.0 centimetre (0.39 in) apart, with one segment to a vertebra.[31] Another tail, this time pertaining to a juvenile E. annectens, had fossilized impressions including tubercles as well as previously unseen skin textures. These impressions included elliptical overlapping scales, grooved scales, and a "9 cm by 10 cm trapezoidal horn-like structure"

Edmontosaurus was a hadrosaurid (a duck-billed dinosaur), a member of a family of dinosaurs which to date are known only from the Late Cretaceous. It is classified within the Saurolophinae (alternately Hadrosaurinae), a clade of hadrosaurids which lacked hollow crests. Other members of the group include Brachylophosaurus, Gryposaurus, Lophorhothon, Maiasaura, Naashoibitosaurus, Prosaurolophus, and Saurolophus.[8] It was either closely related to[34] or includes the species Anatosaurus annectens (alternately Edmontosaurus annectens),[8] a large hadrosaurid from various latest Cretaceous formations of western North America. The giant Chinese hadrosaurine Shantungosaurus giganteus is also anatomically similar to Edmontosaurus; M. K. Brett-Surman found the two to differ only in details related to the greater size of Shantungosaurus, based on what had been described of the latter genus.[35]

While the status of Edmontosaurus as a saurolophine or (="hadrosaurine") has not been challenged, its exact placement within the clade is uncertain. Early phylogenies, such as that presented in R. S. Lull and Nelda Wright's influential 1942 monograph, had Edmontosaurus and various species of Anatosaurus (most of which would be later considered as additional species or specimens of Edmontosaurus) as one lineage among several lineages of "flat-headed" hadrosaurs.[36] One of the first analyses using cladistic methods found it to be linked with Anatosaurus (=Anatotitan) and Shantungosaurus in an informal "edmontosaur" clade, which was paired with the spike-crested "saurolophs" and more distantly related to the "brachylophosaurs" and arch-snouted "gryposaurs".[34] A 2007 study by Terry Gates and Scott Sampson found broadly similar results, in that Edmontosaurus remained close to Saurolophus and Prosaurolophus and distant from Gryposaurus, Brachylophosaurus, and Maiasaura.[33] However, the most recent review of Hadrosauridae, by Jack Horner and colleagues (2004), came to a noticeably different result: Edmontosaurus was nested between Gryposaurus and the "brachylophosaurs", and distant from Saurolophus.[8] The discrepancies are complicated by the relative lack of work on hadrosaurine evolutionary relationships.